Asynchronous transfer mode (ATM) is an asynchronous, high bandwidth, low latency, cell-based switching and multiplexing technology. ATM is designed to provide a general-purpose, connection-oriented transfer mode for a wide range of services. To provide connection-oriented service, ATM employs the concept of a hierarchical logical connection. The logical connection includes a virtual path (VP) that may contain one or more virtual channels (VCs). In addition, multiple streams of packetized data may be multiplexed onto a single VC through the use of a channel identifier (CID) associated with each stream. As illustrated in FIG. 1, ATM also includes multiple layers, such as the physical layer, the ATM layer, and various ATM adaptation layers (AALs) (e.g., ATM adaptation layer 2 (AAL2)) that may interface with other higher layers. Each of the layers may support different services and each may include various sublayers.
The AAL2 layer supports ATM transport of connection-oriented variable bit rate (VBR) packetized data, such as voice data and video data. The AAL2 layer typically communicates with a higher layer to transmit and receive the VBR data, generally in a time division multiplexed (TDM) format. The AAL2 layer also includes a common part sublayer (CPS) that interfaces to the ATM layer.
Data received by the AAL2 layer are encapsulated in CPS packets that are similar to, for example, the packet of FIG. 2A, and that include a 3-octet header and a variable length payload of from 1 to 64 octets. Next, CPS protocol data units (CPS-PDUs) (illustrated with respect to FIG. 2B) are formed from 48-octets of CPS packets. Because of the variable CPS packet size, a CPS-PDU may contain multiple CPS packets, or a single CPS packet may span multiple CPS-PDUs. Once packed, the 48-octet CPS-PDU is communicated to the ATM layer to form a complete payload of an ATM cell, as illustrated by FIG. 2C.
The AAL2 also may receive CPS-PDUs from the ATM layer, from which CPS packets may be extracted and passed up to the AAL2 for processing. The process of packing CPS packets into CPS-PDUs and of extracting packets from CPS-PDUs may be referred to collectively as segmentation and reassembly (SAR).
The ATM layer receives CPS-PDUs from the AAL2 and constructs 53-octet ATM cells from them by adding a 5-octet header and by using the 48-octet CPS-PDU as the ATM cell payload (see, e.g., FIG. 2C). The ATM layer also accepts 53-octet ATM cells from the physical layer and unpacks the CPS-PDUs from them to pass up to the AAL2. Based upon information stored in the cell headers, the ATM cells are multiplexed, switched and controlled by the ATM layer.
Finally, the physical layer accepts cells from the ATM layer and frames them for transmission over an electrical or optical transmission medium. The physical layer also accepts frames from the transmission medium and unpacks the ATM cells from the frames.
Like reference symbols in the various drawings indicate like elements.